The effect of microwave radiation was studied on active secretory cells. Rat peritoneal mast cells were exposed to 2450 MHz microwave radiation at specific absorption rates (SAR) of 8.5 and 42.5 mW/ml for periods of up to 3 hrs. Cells were maintained throughout exposure at 37 degrees C. There was no effect on cell viability or spontaneous histamine release. Mast cells exposed to compound 48/80 after prior irradiation or during simultaneous irradiation secreted histamine in a manner similar to unexposed cells.